A number of methods have been suggested for determining loads acting on a train wheel.
An often used method comprises equipping each wheel with strain gauge arrangements called Wheatstone bridges. Two such bridges are provided for the vertical force and one bridge is provided for the lateral force. The signals are decoupled based on the placement of the strain gauges. The signal conditioning equipment is located on board and connected to the wheel via a slip ring device. Since the vertical force is not measured directly, a piece of analogue hardware, a ‘Q-shaper’, is used to obtain the force from the measured signals. A number of disadvantages are connected to this method. The wheels must have curved web to obtain separation of the lateral and vertical track forces. This means that specially manufactured wheels have to be provided for the test. Further, the in- as well as the outside of the wheel disc has to be instrumented. This means that the wheel discs have to be provided with holes to transfer the signal between the in- and outside. Such operations may compromise the structural integrity of the wheel, shorten its life, and render it un-useful for normal operation after the tests are finalised.
Further known method for determining train wheel loads can be found in, for example, DE3114499A1 and EP580969A1.
DE1079348 describes a method in which two strain gauges are placed on a train wheel, one on each side thereof, and opposite to each other. Calibration values corresponding to relationships between strains and wheel angles are established for the strain gauges at vertical and side loads. Two equations with relationships between measured strains, the calibration values and true loads are provided. Although one embodiment suggests the use of additional strain gauge pairs for a sequential provision of measured values, at any given point in time only two strain gauges, located at the same radial and angular location (in angular coordinates) of the wheel, are used to calculate the loads. This results in the system being sensitive to interference, and noise levels, resulting in a risk of erroneous results. Further, as in the solution described above, the in- as well as the outside of the wheel disc has to be instrumented, resulting in the need for structurally detrimental operations on the wheel.
In general, in most known methods, the output from the gauges will generally be a combination of all features present in the wheel and in the electrical system used to measure it, as well as disturbances. In general, regarding each load to be determined, there is a need to improve the separation of measurements belonging to this load from the rest of the signals.